Method and apparatus for removing impurities from waste water by electroflotation

ABSTRACT

The invention relates to a method and apparatus for removing impurities from waste water by electroflotation. The waste water to be cleaned is conducted through an electrolytic cell. Electrolysis is performed between two electrodes ( 1, 2 ) of different electronegativities, such that the more electronegative electrode ( 1 ), which is non-wearing in a cleaning process, is used for producing hydrogen gas and hydroxyl ions from water. The less electronegative electrode ( 2 ), which is an active, wearing electrode in a cleaning process, is used for producing metal ions in a solution to be cleaned. In addition to this basic reaction, a desired oxidation-reduction reaction is initiated in the cell in a strictly controlled electric field for removing one or more designated impurities from cleaned water.

The invention relates to a method for removing impurities from wastewater by electroflotation, in which method the waste water to be cleanedis passed through an asymmetrical electrolytic cell, resulting in a cellreaction which produces both metal hydroxide and hydrogen gas. If theactive electrode is made of iron or aluminium, the cell reactionproduces iron or aluminium hydroxide, respectively.

The invention relates also to an apparatus for removing impurities fromwaste water by electroflotation.

The term electroflotation is based on the fact that the gas evolving inelectrolytic cells raises also metal hydroxide (typically iron oraluminium hydroxide) produced in the cells, and impurities filteredthereby from water, to the surface of clean water, enabling themechanical removal of flock therefrom. This separation of flock andwater is already initiated in an electrolytic cell and can be completedin a flock extraction tower, such as those described in the Applicant'spatent publications U.S. Pat. No. 5,888,359 and U.S. Pat. No. 6,086,732,or in conventional secondary settling tanks used in sewage treatmentplants.

A problem in sewage treatment has been the lack of means for asufficient removal of harmful impurities, such as nitrogen, and toxiccompounds, such as chlorophenols and polyaromatic hydrocarbons.

The invention relates to a method and apparatus, capable of removingimpurities from waste waters more effectively than before andeconomically.

This object is achieved in the invention by means of a method as setforth in the appended claim 1 and an apparatus as set forth in claim 6.The dependent claims disclose preferred embodiments or applications forthe method and apparatus.

For example, nitrogen can be removed from low-salt waste waters by meansof electroflotation as described herein always to more than 80%,typically to more than 95%, and from almost salt-free waste waters tomore than even 99%, without chemical additives.

On the other hand, seep waters in landfill sites can be cleared of toxicorganic compounds, while reducing the salt content thereof, as well.

The invention will now be described by way of an exemplary embodimentwith reference to the accompanying drawings, in which

FIG. 1 shows one preferred exemplary embodiment for an electrolytic cellpracticable in a method and apparatus of the invention; and

FIG. 2 shows schematically an entire cleaning plant according to onetest system.

The electrodes of an electrolytic cell according to the invention(FIG. 1) are comprised of pipes. An inner electrode pipe 1 is made ofstainless steel and provided with holes 4 for directing washing spraysto the surface of an outer electrode pipe 2 of undoped metal. The metalused for the outer electrode pipe comprises typically iron or aluminum.The cylindrical electrode pipes 1 and 2 are set coaxially and definetherebetween a cylindrical and non-interrupted electrolysis space 5,wherein the waste water is supplied from a duct 6. The negative pole ofa power source is connected to the inner electrode pipe 1 by way of aterminal 11 and the positive pole to the iron or aluminum electrode pipe2 by way of a terminal 12. The inner electrode 1 is made of steel orsome other metal more electronegative than iron or aluminum. Thus, theinner pipe 1 is non-wearing (releasing only electrons) and the iron-madeouter pipe 2 is prone to wearing as it releases iron ions. This is whythe outer pipe 2 is made readily replaceable as will be describedhereinafter.

The inner electrode pipe 1 is divided by a partition 1 a for twoseparate tubular spaces 8 and 9. The tubular space 8 covers essentiallythe length of the electrolysis space 5 and is provided with washingspray holes 4. The tubular space 9 is connected by way of quite largeholes 7 with the downstream end of the electrolysis space 5, the waterand resulting flock being able to flow from the electrolysis space 5into the pipe section 9. The ends of the pipe sections 8 and 9 arefitted with inlet and outlet tubes of an insulating material, such asplastics. The pipe section 8 is supplied with washing water at apressure sufficient for delivering appropriately powerful washing spraysfrom the holes 4. The electrodes' surface can also be cleaned byconducting a pulse of alternating current to the electrodes.

The iron or aluminium pipe 2 terminates prior to the sewage inlet point6 and the inner pipe 1 continues past the inlet point 6 by way of avalve 18 to a wash water pump. The valve 18 has its opening action and awash water pump 19 has its actuation controlled by a control device 20to proceed intermittently. Over each wash cycle, a valve 17 for anoutlet duct 16 coupled with the bottom end of the electrolysis space 5is adapted to be opened for discharging precipitate and wash water fromthe electrolysis space 5.

The outer electrode 2 is further encircled by a housing tube 3 of aninsulating material, such as plastics.

The electrolytic cell is held together by end caps 10 and 15. In theillustrated case, the pipe 2 has its ends provided with male threadsengaged by threads of the end caps 10 and 15. Upon tightening the endcap 10, a packing 13 is pressed by conical surfaces 14 against the outersurface of the inner pipe 1. In so doing, the packing 13 also compressesagainst the end surface of the outer pipe 2. The electrolysis space 5has its bottom end sealed with a packing which is pressed against aninner shoulder of the bushing 15 by means of a plug 15 a. The end caps10 and 15 retain the pipes 1 and 2 concentrical relative to each other.Of course, the electrode pipes can be provided with end assemblies otherthan what is described above.

The pipes 1 and 2 can be supplied in diameters and lengths varyingaccording to a particular application. As the size of a processing plantbecomes larger and flows increase, a sufficient number of cells will beconnected in parallel.

Using the concentrically nested electrode pipes 1 and 2, as well as theflushing spray holes 4 in the inner pipe 1, provides a simple way ofmaintaining a clean electrode surface. By virtue of the unscrewable endcaps 10 and 15, the wearing iron or aluminium electrode pipe 2 isreadily replaceable.

The following discloses foundations, which constitute the basis for amethod of the invention for denitrification by electroflotation. Theactive electrode 2 is made of iron.

1. Cell Reactions

1.1. H₂O

H⁺+OH⁻

1.2. Fe

Fe⁺³+3e⁻

1.3. Fe⁺³+3OH⁻

Fe(OH)₃↓(iron hydroxide)

1.4. 2H⁺2e⁻

H₂↑(hydrogen gas)

The electrolysis produces a mildly alkaline solution, since H⁺ ionsescape as hydrogen gas from the solution more quickly than OH⁻ ions.

2. Removal of Nitrogen

A. Ammonium (NH₄ ⁺) Nitrogen:

2.1. NH₃+H₂O

NH₄ ⁺+OH⁻

2.2. NH₃+OH⁻+H⁺

NH₄ ⁺+OH⁻

2.3. NH₃+H⁺

NH₄ ⁺ (ammonium ion)

In electrolysis, the H⁺ ion bonds with an ammonia molecule and forms anammonium (NH₄ ⁺) ion. It does not evaporate, but dissolves in water.When the aqueous solution contains e.g. an SO₄ ²⁻ ion, the electrolysisserves to remove an NH₄ ⁺ ion and nitrogenous organic substances whichcoprecipitate with iron hydroxide. The precipitate rises along with H₂gas as a flock to the surface of clean water. Prior to its passage intoa electrolytic cell, the waste water could have been supplied in aconventional manner e.g. with an appropriate amount of acidic ferroussulphate.

2.4. Fe(OH)₃+SO₄ ²⁻+2NH₄ ⁻⁺+R

R—Fe(OH)₃↓+(NH₄)₂SO₄

In electrolysis, the NH₄ ⁺ nitrogen present in waste water and organicnitrogenous compounds (R) coprecipitate into iron hydroxide precipitateFe(OH₃)↓ or the NH₄ ⁺ nitrogen may also be reduced while iron oxidizesto iron oxide.

2.5. Fe+NH₄ ⁺+OH⁻

FeO+2½H₂↑+½N₂↑

2.6. 2NH₄ ⁺+2e⁻

N₂↑+4H₂↑

2.7. Fe

Fe²+2e⁻

The H₂ gas (hydrogen gas), evolved simultaneously in the cell, raisesthe precipitate as a flock to the surface of clean water in a flockextraction tower and/or in a secondary settling tank. Thus, nitrogen isremoved in a solid form. (Operation of a flock extraction tower isdescribed in patent publications U.S. Pat. No. 5,888,359 and U.S. Pat.No. 6,086,732).

NH₄ ⁺ nitrogen develops in sewage waters principally from urea asfollows:

2.8.

2.9. NH₃+H₂O

NH₄ ⁺+OH⁻B. Nitrate (NO₃ ⁻) Nitrogen:

Ammonia is oxidized by microbes to nitrate (nitrification) or toaminonitrogen, which bonds primarily inside microbe cells in abiochemical reaction promoted by enzymes (enz.).

2.10.

This is a sum reaction. The intracellular reaction is enzymaticallycatalyzed and much more complicated.

2.11.

In electrolysis, iron is oxidized in cells (always) and nitrogen (NO₃ ⁻)is reduced as follows:

2.12. 6Fe+2H⁺+2NO₃ ⁻

6FeO↓+H₂↑+N₂↑

2.13. 2Fe+H⁺+NO₃ ⁻

Fe₂O₃↓+½H₂↑+½N₂

-   -   =>NITROGEN ESCAPES FROM WASTE WATER AS NITROGEN GAS (N₂)        (denitrification)

Microbe cells also establish denitrification in anaerobic conditions(without oxygen), in which the NO₃ ⁻ ion functions as an oxidant insteadof the O₂ molecule.

-   -   Denitrification carried out by electrolysis in cells is nearly        quantitative and really quick as compared to a slow and more        expensive removal of nitrogen effected by means of microbes.    -   Biological denitrification as a nitrogen elimination method        provides a nitrogen reduction of about 63% with comparatively        expensive technology.    -   Electrolysis has always provided a nitrogen reduction of more        than 80%, and at best, e.g. in the treatment of cow dung, a        nitrogen reduction of more than 99%, such that the nitrogen        content of cleaned water is less than 2 mg/l.

The oxidation of iron to a ferric or ferrous ion and the reduction ofnitrogen take place in a cell at a certain point of resonance energy, Inother words, the electrical energy introduced into a cell must bedimensioned according to the dimensioning and flow of the cell, i.e. theretention time of waste water in the cell space. The search for a properpoint in resonance energy must be conducted experimentally and then thecell flow is controlled by automation with respect to the flow of wastewater. The flow-through of waste water need not be intercepted for awashing period, since the washing is executed at a substantially higherpressure and with a smaller liquid volume than the pressure and liquidvolume of through-flowing waste water.

3. Treatment of Landfill Seepage

An apparatus as shown in FIG. 2 was used for conducting a series oftests in relation to the applicability of the apparatus for the cleaningof landfill seep water.

The following describes first a test apparatus, then a test procedure,and finally test results.

3.1 Test Apparatus

-   -   A two-stage apparatus in the sense that two electrolytic cells        28 provided with aluminium electrodes (or two iron electrodes or        one iron and one aluminium electrode) are successively in        cascade connection. One test was conducted in a single stage and        the other test in a double stage. From a tank 21 seepage is        pumped with a pump 27 through an electrolytic cell 28. From a        tank 22 a polymer solution is fed by a pump 29 into a mass flow        discharging from the cell 28, which is conveyed to a flock and        purified water separation tower 30, the top end of which        comprises a measurement 31 for outgoing gases (HCl, Cl₂). From        the bottom end of the tower 30 purified water is conducted into        a tank 23 and from the top end of the tower 30 flock is        conducted into a tank 24. In the second stage, purified water is        delivered from the tank 23 by means of a pump 27 through a        second electrolytic cell 28 to a second flock and purified water        separation tower 30. Also, in the second stage, a mass flow        between the cell 28 and the tower 30 is supplied with a polymer        solution by a pump 29 from a tank 25. The twice-treated water is        conducted from the bottom end of the tower 30 into a tank 26 and        the flock is conducted also from the top end of the second tower        30 into the tank 24.        3.2 Test Procedure (Test Runs)    -   10 separate test runs were conducted, some in a single stage        with both Fe- and Al-cells. Some of the two-stage test runs were        conducted by using two different cell types, namely an Fe-cell        (active electrode of iron) and an Al-cell (active electrode of        aluminium).

The following describes two interesting and representative test runs.

Test Run 1

Test involved a two-stage purification treatment. The first stageinvolved the use of an Al-cell and the second stage an Fe-cell. Thefirst stage involved feeding undiluted seep water through the Al-cell ata rate of about 120 l/h. Polymer solution was fed from the tank 22 bythe pump 29 at a rate of 10-12 l/h. Power was supplied to the cell overthe current range of 10-50 A and the voltage range of 3-7 V. The entirepower range was subjected to scanning, and the finding was that theclarification and decoloration of solution were directly proportional toelectric power. At a power of more than 1 kWh/m³, the formation of flockno longer improved. Gas analysis was conducted at an electric powerwhich corresponds to about 1.0 kWh/m³ of undiluted seepage. Formation ofchlorine gas was not observed. The second stage involved delivering theclean water fraction of the first stage through an Fe-cell at a feedrate of 60-120 l/h. Polymer solution was fed at a rate of 10 l/h.Scanning was conducted across the entire power range at a current of10-30 A and a voltage of 3-7 V.

Test Run 2

The undiluted seep water was run twice through an Al-cell. Feed rate was60-150 l/h of waste water and 10-15 l/h of polymer solution. Averagepower supply for the cell was 30 A, 3 V. Quick scanning was alsoconducted to a maximum power of 52 A, 7 V. Gas formation was powerfuland flock climbed very quickly in the flock separation tube 30, in whichthe separation surface of flock remained easily stationary (could bemonitored through the clear tube). Water coming from the tower 30clarified at settings as low as 30 A/3 V/120 l/h, i.e. at a power of0.75 kWh/m³. Increasing power beyond 1 kWh/m³ provided no improvement.Formation of chlorine gas was not observed.

3.3 Analytical Data from Test Runs

-   -   Samples picked up from clean water fractions and flock fractions        were analyzed in different laboratories by standard methods.        Samples were analyzed for more than 80 chemical parameters, only        the most important of which will now be dealt with in a        summarized manner.    -   The seep water to be cleaned had a yellow-brownish and somewhat        cloudy appearance. Judging from odour, it contained ammonium and        sulphur compounds. Both tests confirmed that seep water        clarifies and becomes almost colourless and odourless. The first        test's firs stage was intentionally conducted to produce only        partially purified water by optimizing the process just right        only in terms of flock formation, using electric power as little        as possible. The second test's first stage was conducted by        imitating the first test's first stage. The second test's second        stage was conducted with intention to set a balance for a final        result as clean as possible.    -   Both tests showed a substantial reduction in electrical        conductance. In the second test, a reduction in conductance was        about 30%. Conductance in flock is substantially higher than in        cleaned water, i.e. flock has conductance promoting compounds        concentrated therein. Ph remained approximately unchanged.    -   Nitrogen reduction remained at a level lower than in the process        of cleaning other lower-salt waste waters. The tests indicated        that the removal rate of nitrogen was substantially consistent        with the reduction of salt content.    -   Phosphorus was eliminated in cleaning almost completely, even at        high concentrations.    -   Formation of chlorine or hydrogen chloride was not observed in        test conditions, although the seep water to be cleaned was in        fact a saline solution. On the other hand, the reduction of        chloride by about 29%, and its concentration in flock, suggests        that there could be some compound bonding a chloride ion to        flock. The conclusion is that chloride must have bonded to        flock, e.g. by way of substitution reactions to organic        molecules, or in the form of a salt to flock-forming, very        high-density ferric hydroxide precipitate which functions as a        molecular sieve.        Salt Content and Removal of Ammonium Nitrogen

Calculations based on concentrations of chloride and sodium ionsresulted in a salt content of about 3.6%. In single-stage cleaning thesalt content fell to about 2.9%, i.e. the reduction was about 19% (at apower of 1 kWh/m³).

In two-stage cleaning the salt content fell to about 2.4%, i.e. thereduction was about 33% (at a power of 1.75-2 kWh/m³).

Within the margin of error, these results provided a conclusion that theremoval of salt is almost linear with respect to applied power.

The Fe-cell was able to eliminate 47% of salt, i.e. the salt contentfell to the level of 1.92%, at a power of 3 kWh/m³.

The results provided a conclusion that about 3 kWh/m³ would besufficient to remove about 50% of salt (to salt content of 1.8%) andabout 6 kWh/m³ would be enough to eliminate salt completely from seepwater.

In light of the above, the method and apparatus are highly suitable forcleaning generally salt-containing waste water, such as contaminated seawater.

In this conjecture, it should be noted that the removal of ammoniumnitrogen from clarified water is proportional to the variation of saltcontent in seep water or other such waste water in the described testconditions.

The reductions of ammonium nitrogen and salt seem to correlateperfectly, it being also confirmed experimentally that ammonium nitrogencan be removed from waste water by 99% (to 10 mg/l from 1100 mg/l), asthe waste water has a salt content of less than 0.8%.

Heavy metals were removed from seep water to flock so efficiently thatno presence thereof was observed in cleaned water.

Phenols and Chlorophenols

The reduction of phenols was over 90%. About 80% of phenols haddispersed in electroflotation and a small amount with respect to cleanedwater had concentrated in flock.

Chlorophenols were removed by 100% from cleaned water. Chlorophenolshave dispersed in a cleaning process by about 90%. Only 10% of theiroriginal amount in salt water was found in flock. The most interestingobservation is pentachlorophenol, which had disappeared completely in acleaning process. The observation is consistent with previous testresults. The likely reason is the scission of a benzene ring.

Polyaromatic Hydrocarbons (PAH)

No PAH compounds were found in cleaned water. Reduction is 100%. Of allPAH compounds, over 94% had dispersed during a cleaning process.

Summary of Test Results

On the basis of test results, seep water is most preferably cleaned withan apparatus, comprising a cascade made up by an Al-cell and an Fe-cell.Cleaning can also be managed with an Fe-cell alone, provided that seepwater does not contain large amounts of sulphides. Cleaning can be donewith an Al-cell alone, but the cleaning cost will be considerably higherthan with an Fe-cell.

Due to fluctuations regarding the salt water composition, it isadvisable to link the cells in such a way that cleaning can be effectedeither by just one type of cell or by a combination of two cell types.

According to measurements, the lowest practical electric power is about3 kWh/m³ seep water and the maximum electric power for a cleanest resultis not higher than 6 kWh/m³ seep water.

A preferred choice for the non-wearing electrode is steel as the amountsof its alloy metals can be used to regulate how much theelectronegativity increases with respect to iron. Aluminium has a lowerelectronegativity than iron.

Thus, the selection of metals for active electrodes can be used toinfluence a electronegativity difference. It is sufficient that theelectrodes be coated with metals whose electronegativity difference isappropriate for a material to be eliminated for accomplishing itsremoval based on a redox or oxidation-reduction reaction.

1. A method for removing impurities from waste water byelectroflotation, the method comprising the steps of: passing the wastewater to be cleaned through an electrolytic cell (28) provided with twometal electrodes (1,2) of different electronegativities; performingelectrolysis between the two electrodes (1,2), such that the moreelectronegative electrode (1), which is non-wearing in a cleaningprocess, is used for producing hydrogen gas and hydroxyl ions fromwater, and that the less electronegative electrode (2), which is anactive, wearing electrode in a cleaning process, is used for producingmetal ions in a solution to be cleaned, the method further comprising acombination of following steps: automatically controlling the cellcurrent to obtain a controlled electric field in the cell; effecting inthe cell in the controlled electric field a desired oxidation reductionreaction for removing one or more designated impurities from water to becleaned; feeding the mass flow from the cell to a separation tower (30)that separates a floc and purified water and allowing the hydrogen gasto raise the floc in the separation tower (20); using coaxial pipes aselectrodes, the coaxial pipes defining an uninterrupted, generallycylindrical electrolysis space in the cell through which the hydrogengas raises the floc along a generally linear path that is notinterrupted by structural elements that cause mixing in the cell so thehydrogen gas raises the floc through the electrolysis space withoutmixing the floc with the purified water so as to allow the formation ofthe floc, the inner electrode pipe being the more electronegativeelectrode (1), having holes; and feeding flush water intermittently,during wash cycles, through the inner electrode pipe by pressure forproducing flush water sprays through the holes against inner surface ofthe outer electrode pipe.
 2. A method as set forth in claim 1 forremoving nitrogen from waste water, characterized in that: (a) inelectrolysis, hydrogen ions (H.sup.+) are used for producing fromammonia (NH.sub.3) ammonium ions (NH.sub.4.sup.+), which escape uponjoining negative ions and upon coprecipitating with iron hydroxideprecipitate; (b) the precipitate is allowed to rise along with hydrogengas in the form of floc to the surface of clean water in the flocseparation tower (30); and (c) in electrolysis, iron is oxidized andNH.sub.4.sup.+ nitrogen and/or nitrate nitrogen (NO.sub.3) is reduced asfollows:Fe+NH₄ ⁺OH⁻

FeO↓+2H₂↑+½N₂↑and/or6Fe+2H⁺+2NO₃

6FeO↓+N₂↑+H2↑ whereby the result is denitrification as nitrogen escapesfrom waste water in the form of nitrogen gas.
 3. A method according toclaim 1, where the waste water is landfill seepage or some othersalt-containing waste water.
 4. A method according to claim 3,characterized in that the seepage or other salt-containing waste waterto be cleaned is conducted in a first stage through a first electrolyticcell, and in a second stage the water partially cleaned in the firststage of conducted through a second electrolytic cell.
 5. A method asset forth in claim 1, characterized in that the less electronegativeelectrode is made of iron or aluminum.
 6. An apparatus for removingimpurities from waste water by electroflotation, said apparatuscomprising a set of electrolytic cells, each cell thereof being providedwith one or more metal electrodes (2) coupled with a positive pole of apower source and one or more metal electrodes (1) coupled with anegative pole of the power source, and an electrolysis space (5) betweenthe electrodes, the electrode (1) connected to the negative pole of apower source being made at least in its surface layer from a moreelectronegative material than the electrode (2) connected to thepositive pole, the more electronegative electrode (1) being non-wearingin a cleaning process and releasing only electrons received thereby intoa solution to be cleaned, and the less electronegative electrode beingan active, wearing electrode in a cleaning process and releasing metalions into a solution to be cleaned, the electrodes (1, 2) having such anelectronegativity difference that a desired oxidation-reduction reactionis achieved; a control device controlling a cell current, therebyenabling a desired oxidation-reduction reaction in the cell; aseparation tower (30) separating a floc and purified water, allowinghydrogen gas to raise the floc in the separation tower (30); a pump (27)for pumping a mass flow through the cell (28), as a closed continuousflow, to the separation tower (30); coaxial pipes as the electrodes (1,2), the inner electrode pipe being the more electronegative electrode(1) and having holes (4), the coaxial pipes defining an uninterruptedcylindrical electrolysis space, uninterrupted by structural elementsthat cause mixing of the floc and purified water as the hydrogen gasraises the floc in the separation tower; and flushing means (16-20) forfeeding flush water intermittently, during wash cycles, through only theinner electrode pipe by pressure for producing flush water spraysthrough the holes (4) against inner surface of the outer electrode pipe(2), the flushing means feeding the flush water, during the wash cycles,at a pressure higher than a pressure at which the pump (27) introducesthe mass flow to the separation tower.
 7. An apparatus as set forth inclaim 6, characterized in that the less electronegative electrode ismade of iron or aluminum, the iron or aluminum pipe (2) being theoutermost and readily replaceable.
 8. An apparatus as set forth in claim7, characterized in that the outer electrode pipe (2) terminates priorto a waste water inlet (6), while the inner pipe (1) continues past thewaste water inlet (6) by way of a valve (18) to a wash water pump (19).9. An apparatus as set forth in claim 8, characterized in that the valve(18) has its opening and the wash water pump (19) has its actuationcontrolled to proceed intermittently, while a valve (17) in an outletduct (16) connected to the bottom end of the electrolysis space (5) isadapted to be opened for discharging precipitate and wash water from theelectrolysis space (5).
 10. An apparatus as set forth in claim 7,characterized in that the inner electrode pipe (1) is made of stainlesssteel and the iron- or aluminum-made outer electrode pipe (2) is coveredwith an insulating housing tube (3).
 11. An apparatus as set forth inclaim 7, characterized in that the electrode pipes (1, 2) are lockedconcentrically to each other by means of unscrewable end caps (10, 15),which surround the ends of the inner electrode pipe (1) and inside whichare retained the ends of the outer electrode pipe (2).
 12. A method asset forth in claim 2, characterized in that the less electronegativeelectrode is made of iron or aluminum.
 13. A method as set forth inclaim 3, characterized in that the less electronegative electrode ismade of iron or aluminum.
 14. A method as set forth in claim 4,characterized in that the less electronegative electrode is made of ironor aluminum.
 15. An apparatus as set forth in claim 8, characterized inthat the inner electrode pipe (1) is made of stainless steel and theiron- or aluminum-made outer electrode pipe (2) is covered with aninsulating housing tube (3).
 16. An apparatus as set forth in claim 9,characterized in that the inner electrode pipe (1) is made of stainlesssteel and the iron- or aluminum-made outer electrode pipe (2) is coveredwith an insulating housing tube (3).
 17. An apparatus as set forth inclaim 8, characterized in that the electrode pipes (1, 2) are lockedconcentrically to each other by means of unscrewable end caps (10, 15),which surround the ends of the inner electrode pipe (1) and inside whichare retained the ends of the outer electrode pipe (2).
 18. An apparatusas set forth in claim 9, characterized in that the electrode pipes (1,2) are locked concentrically to each other by means of unscrewable endcaps (10, 15), which surround the ends of the inner electrode pipe (1)and inside which are retained the ends of the outer electrode pipe (2).19. An apparatus as set forth in claim 10, characterized in that theelectrode pipes (1, 2) are locked concentrically to each other by meansof unscrewable end caps (10, 15), which surround the ends of the innerelectrode pipe (1) and inside which are retained the ends of the outerelectrode pipe (2).
 20. An apparatus as set forth in claim 15,characterized in that the electrode pipes (1, 2) are lockedconcentrically to each other by means of unscrewable end caps (10, 15),which surround the ends of the inner electrode pipe (1) and inside whichare retained the ends of the outer electrode pipe (2).
 21. A methodaccording to claim 3, wherein the salt-containing waste water iscontaminated sea water.
 22. A method for removing impurities from wastewater by electroflotation, the method comprising the steps of: passingthe waste water to be cleaned through an electrolytic cell (28) providedwith two metal electrodes (1,2) of different electronegativities;performing electrolysis between the two electrodes (1,2), such that themore electronegative electrode (1), which is non-wearing in a cleaningprocess, is used for producing gas and ions from water, and that theless electronegative electrode (2), which is an active, wearingelectrode in a cleaning process, is used for producing metal ions in asolution to be cleaned, the method further comprising a combination offollowing steps: automatically controlling the cell current to obtain acontrolled electric field in the cell; effecting in the cell in thecontrolled electric field a desired oxidation reduction reaction forremoving one or more designated impurities from water to be cleaned;feeding the mass flow from the cell to a separation tower (30) thatseparates a floc and purified water and allowing the gas to assist inraising the floc using coaxial pipes as the two electrodes, the coaxialpipes defining, uninterrupted by structural elements that cause mixingof the floc and purified water, an uninterrupted, generally cylindricalelectrolysis space through which the gas raises the floc in the cellalong a generally linear path, the inner electrode pipe being the moreelectronegative electrode (1); and intermittently cleaning the coaxialpipes by injecting flush water during wash cycles, the wash cycles beingintermittent to permit formation of the floc and to permit the gas toraise the floc in the cell.
 23. The method of claim 22 wherein there areonly two electrodes and wherein the less electronegative electrodecomprises iron.
 24. The method of claim 22 wherein there are only twoelectrodes and wherein the less electronegative electrode comprisesaluminum.